1. Field of the Invention
The present invention relates to a structure for introducing secondary air into an exhaust path of an internal combustion engine. More particularly, the present invention relates to a structure for introducing secondary air into an exhaust path of an internal combustion engine by which the preheating effect of introduced secondary air is improved and the re-combustion property of exhaust gas is enhanced.
2. Description of the Related Art
Conventionally, it is well-known that secondary air is introduced into an exhaust path of an internal combustion engine and re-combusted in the exhaust path, so that HC and CO contained in exhaust gas, which are unburned components, are reduced. In this case, at a position where the temperature of the exhaust gas is high, re-combustion of exhaust gas is more effectively carried out. Therefore, the secondary air is introduced into a portion of the exhaust path which is located as close to an exhaust port as possible.
In the secondary air introducing structure from which secondary air is introduced into the exhaust path of the internal combustion engine shown in FIG. 2, there is formed an exhaust path 105 in a cylinder section 100 composed of a cylinder block 102 and a cylinder head 101. In the periphery of the exhaust path 105, there are formed water jackets 119, 120, 121, and secondary air is introduced into the exhaust path 105.
The water jacket 120 is formed by a water jacket cover 130 attached to an outer face of the cylinder section 100. Outside the water jacket cover 130, there is provided a valve cover 131. By the water jacket cover 130 and the valve cover 131, a valve chamber 132 is formed. In the valve chamber 132, there is provided a valve 133.
In the water jacket cover 130, there is formed an opening 134. In the cylinder section 100, there is formed a communicating path 135 which is communicated with the opening 134. The valve chamber 132 and the exhaust path 105 are communicated with each other by the opening 134 and the communicating path 135 (shown in Japanese Patent Unexamined Publication No. Hei. 5-79325).
Accordingly, after secondary air has been introduced from an air tube 136 into the valve chamber 132, it passes through a valve 133 when the valve 133 is opened by the pulsation effect of exhaust gas. Then, the secondary air is introduced into the exhaust path 105 via the opening 134 and the communicating path 135. In the exhaust path 105, exhaust gas is subjected to re-combusting by the secondary air.
The conventional secondary air introducing structure by which secondary air is introduced into the exhaust path of the internal combustion engine is composed as described above. Accordingly, the water jackets 120, 121 are arranged between the valve 133 and the exhaust path 105. Heat of exhaust gas flowing in the exhaust path 105 is intercepted by these water jackets 120, 121. Therefore, no heat of exhaust gas is transmitted to the valve 133 side. As a result, it is possible to prevent the valve 133 from being affected by heat.
However, the following problems may be encountered in the above conventional secondary air introducing structure from which secondary air is introduced into the exhaust path of the internal combustion engine. In the above conventional secondary air introducing structure, the secondary air introducing path, which is composed of the valve chamber 132, opening 134 and communicating path 135, is formed being protruded onto the side of the cylinder section 100. Therefore, the path structure is complicated. Further, the exhaust port of secondary air, which is an opening of the communicating path 135 on the exhaust path 105 side, is distant from the exhaust valve 107. Therefore, secondary air can not be preheated to a sufficiently high temperature. Accordingly, the temperature of mixture, in which exhaust gas is mixed with secondary air, is not raised to a sufficiently high temperature. As a result, the function of a catalyst is lowered, and re-combustion of exhaust gas is not sufficiently carried out.